1. Field of the Invention
The present invention generally relates to wellbore construction. More particularly, the invention relates to an apparatus and method for deactivating a downhole tool. More particularly still, the invention relates to an expandable bit with a secondary release device.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. The drill bit generally includes a body portion for securing the drill bit to the drill string and a crown portion to form the wellbore. After drilling a predetermined depth, the drill string and the drill bit are removed, and the wellbore is lined with a string of steel pipe called casing. The casing typically includes a smaller outside diameter than the drill bit that formed the wellbore. The casing provides support to the wellbore and facilitates the isolation of certain areas of the wellbore adjacent hydrocarbon bearing formations. The casing typically extends down the wellbore from the surface of the well to a designated depth. An annular area is thus defined between the outside of the casing and the earth formation. This annular area is filled with cement to permanently set the casing in the wellbore and to facilitate the isolation of production zones and fluids at different depths within the wellbore.
In a conventional completion operation, it is common to employ more than one string of casing in a wellbore. In this respect, the well is drilled to a second designated depth of a smaller diameter, and a second string of casing, or liner, is run into the drilled out portion of the wellbore. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing and then cemented in place. This process is typically repeated with additional casing strings until the well has been drilled to a total depth. As more casing strings are set in the wellbore, the casing strings become progressively smaller in diameter in order to fit within the previous casing string. In the conventional completion operation, the drill bits must be progressively smaller as the diameter of each casing string decreases in order to fit within the previous casing string.
From time to time, for a variety of reasons it is necessary to form a portion of a wellbore that is at least as large as the section of the cased wellbore thereabove. For example, a monobore well consist of a sequence of expandable liners that are run through the existing casing, then expanded to achieve the same post-expansion through-bore. In forming the monobore well, the portion of the wellbore below the cased portion must be at least as large as the section of the cased wellbore thereabove.
There are a variety of different methods of forming an enlarged wellbore. One such method is by positioning a conventional under-reamer behind the drill bit to cut the enlarged wellbore. In this drilling configuration, the drill bit acts as a pilot bit to cut the inner cross-sectional area while the under-reamer enlarges the cross-sectional area. Generally, the conventional under-reamer includes a number of expandable arms that move between a closed position and an open position. The ability of the conventional under-reamer to open and close the arms allows the under-reamer in the closed position and the pilot bit to travel though a smaller diameter casing. After passing through the casing the underreamer may be opened to form an enlarged diameter bore below the casing shoe resulting in a wellbore equal to or larger than the original drilled hole. Thereafter, the enlarged wellbore may be lined with expandable liners. This procedure of forming the enlarged borehole, although effective may be time consuming and expensive.
In recent years bi-center bits have been developed as an alternative to the conventional under-reamer. Generally, the bi-center bit includes offset cutting members mounted at irregular intervals around the crown of the bit. As the bi-center bit is rotated, the offset cutting members rotate to form an enlarged wellbore. Although, this method of forming an enlarged wellbore is becoming more common the bi-center bits are unstable due to their irregular structure and tend to be more difficult to control for directional purposes than ordinary drill bits. Additionally, the bi-center bits may not drill the expected swept diameter of the offset pads which ream the pilot hole created by the crown.
More recently, an expandable bit has been used to form an enlarged portion of the wellbore. The expandable bit was introduced to over come the deficiencies in the conventional under-reamer and the bi-center bit. An example of an expandable bit is disclosed in International Publication Number WO 01/81708 A1, which is incorporated herein in its entirety. Similar to the conventional under-reamer, the expandable bit includes a set of blades that move between an open position and a closed position. Generally, hydraulic fluid flows through the center of the expandable bit controls the movement of the blades between the open and the closed position. A more detailed discussion of the expandable bit will be described in subsequent paragraphs.
Even though the expandable bit overcomes many of the deficiencies in the conventional under-reamer and the bi-center bit, a problem still exists with the use of the expandable bit to form an enlarged wellbore. The problem includes the possibility that the expandable bit will become stuck in the open position due to some unforeseen event, like a failure in the hydraulic fluid flow or debris that causes the blades to become jammed. For example, the hydraulic fluid used to operate the tool may contain debris or other small particles intermixed with the fluid portion. As the hydraulic fluid flows through the expandable bit, the debris builds inside the tool and eventually may affect the closing of the expandable bit.
The problem results in the expandable bit being stuck downhole because the expandable bit cannot travel through the casing in the open position. When this problem occurs, an operator has several options, however, each option has significant drawbacks. One option is to remove the cemented casing string to access the stuck expandable bit. This option is very time consuming and costly. Another option is to cut the drill string and leave the stuck expandable bit downhole. Thereafter, the operator may drill around the stuck expandable bit or “side track” the well. Although this option is less destructive than the previous option, drilling around an obstruction requires special downhole tools that may not be available at the wellsite. Another option is to mill through the stuck expandable bit. This option is problematic because the expandable bit is constructed from hardened material, resulting in a difficult milling operation that requires replacing the mill tool multiple times.
In view of the deficiency of the expandable drill bit, a need therefore exists for an expandable bit with a release device to shift the blades from the open position to the closed position in the event of a primary means of closing the blades is unworkable. There is a further need for an expandable bit with a release device that allows the expandable bit to move to the closed position in the event that debris forces the blades to remain open. There is yet a further need for an improved expandable bit.